1. Field of Invention
The present invention relates to a high-resolution photolithographic method for forming nanostructures, in particular in the manufacture of integrated electronic devices.
2. Discussion of the Related Art
In the manufacture of semiconductor integrated devices, the need to reduce the overall dimensions of the devices is continuously felt. To this end, the electronic industry is seeking solutions that enable improvement of the photolithographic process, which considerably affects these dimensions.
As is known, lithography is a process that enables transfer of a configuration or “pattern” from a mask to a thin layer of material sensitive to radiation (referred to as “resist” or “photoresist”), which coats a body to be patterned, for example a silicon substrate.
In particular, lithography exploits the capacity of the resist to change its chemical and/or physical properties when exposed to radiation and thus to be selectively removable where it is exposed or not exposed.
To this end, as shown in FIG. 1, a substrate 1 is coated with a resist layer 2; then the resist layer 2 is heated at moderate temperatures (soft bake, SB) to cause evaporation of the residual solvent and improve adhesion between the resist and the substrate, and is exposed to the radiation 5 generated by a light source 3 through a mask 4 previously provided and including zones transparent to radiation and opaque zones, corresponding to the pattern that is to be transferred. After exposure, the resist layer is, in general, baked (post-exposure bake, PEB) on a hot-plate or in an oven. This treatment has multiple functions that depend upon the nature of the resist used.
The resist layer exposed to light hardens (polymerization phenomenon) and withstands subsequent washing with solvents or acids. At this point the wafer is immersed in a solution that develops the pattern transferred to the photosensitive material by exploiting the variations of solubility generated by radiation. According to the type of polymer used, the exposed zones (positive resist) or the non-exposed zones (negative resist) can be removed.
Positive photoresists include of some components listed below:
1. Polymeric resin with molecular weight in the region of 1-10 kDa;
2. Photoactive organic molecules (PACs);
3. Levelling agents (SLAB), which prevent formation of undulations on the surface of the resist during spinning;
4. Optional colorants for reducing the effects due to reflectivity of the substrate (standing waves);
5. Spinnable solvent or mixture of solvents.
Since the two most important components are resin and PAC, these resists are, in general, referred to as “two-component resists”.
The polymeric resin most commonly used in the formulation of positive photoresists is the novolac resin, obtained from condensation of cresol with formaldehyde via acid catalysis (see FIG. 2 for the chemical formula of novolac).
Positive resins are soluble in basic aqueous solutions, and their dissolving rate depends upon their molecular weight and the relative proportions of the ortho, meta, and para isomers. The photoactive organic compounds added to the formulation of the resist act as inhibitors of dissolution of the resin in a basic environment.
Negative photoresists are two-component resists like positive photoresists. The operating mechanism of the resist is in any case different. In this case, the photoactive species is a photoreticulating diatide of generic formula N3—X—N3, where X is an aromatic group. The diatide effectively absorbs visible light and forms an extremely reactive nitrene, N—, which is able to introduce itself into the C—H and C═C bonds of the rubber resin in the resist, bringing about a cross-linking between the resin molecules. The cross-linking determines a considerable increase in the molecular weight, with consequent decrease of the solubility of the resist in the zones exposed.
The patterned resist layer is then used as a mask for the subsequent etching process, which enables selective removal of the zones of the film not protected by the resist, thus obtaining transfer of the pattern. The resist layer is finally removed, and the entire process is repeated when it is desired to produce multilevel devices.
The generation of increasingly smaller etched structures has required progressively shorter wavelengths. However, current lithographic techniques are by now reaching the limit. By using light in the deep-ultraviolet (DUV, approximately 248 nm) range it has been possible to obtain etched features of between 150 nm (0.15 μm) and 120 nm (0.12 μm) at most.
However, it is foreseeable that in the near future the market will require integrated devices with characteristics of 100-70 nm, which can be produced using deep-ultraviolet light with wavelengths of 193-157 nm. Beyond that point, wavelengths in the extreme-ultraviolet (EUV) range will be necessary. At these wavelengths, light is, however, absorbed instead of being transmitted by conventional refractive optical lenses that focus the image of the mask on the substrate, rendering polymerization of the photoresist impossible using current technologies.
Methods have thus been developed based upon a progressive reduction of the wavelength of the incident light, as well as upon the improvement of the corresponding materials and of the entire technological process. This research has given rise to as many technologies, among which:                extreme-ultraviolet lithography, high-power laser system (a few kilowatts), 13-nm wavelength, which enables emission of radiation in a single direction, with a high resolution of the image obtained, but which has the drawbacks of being very costly and leaving plasma detritus on the substrate;        x-ray lithography, which has a wavelength of 1 nm, is not affected as the previous method by contamination from dust or particles, but has a resolution limited to 100 nm;        electron-beam lithography, which has a resolution limited by the properties of the resist, not by the wavelength; in addition, the manufacturing process is slow with sequential writing; and        ion-beam projection lithography, which has a very high resolution, and markedly reduces the manufacturing times because the process is controlled by a computer, but is, on the other hand, very costly and requires a reliable ion source.        
The development of nanotechnologies on scanning-probe microscopes has enabled development of nanolithographic techniques based upon spatially selective removal of a polymer or upon local deposition/formation of molecules in the desired zones. A significant example is the so-called dip-pen nanolithography, whereby the tip of an atomic-force microscope (AFM) is covered by molecules, such as thiols, which are able to chemically react with a surface of gold forming strong covalent bonds therewith. By controlling the movement of the tip on the surface it is possible to exploit a drop of water as a channel for causing migration of the molecules from the tip to the specimen, obtaining a process similar to dip-pen writing. However, apparatuses of this type are still very costly.
In addition to the problem of miniaturization of the devices, the lithography currently in use in the electronic industry also has the problem that the light rays are reflected by the substrate, generally of silicon, and generate standing waves, as in optical lithography. In addition, scattering phenomena are present both in the resist (forward scattering) and in the substrate (back scattering), which are due to the high energy of the photons emitted by the light beam, in the case of electron-beam lithography. In these cases, proximity corrections are, in general, necessary.
In the latter case, in particular, the photons emitted by the light source, by penetrating in a certain part of the substrate, can cause avalanche generation of free electrons in the substrate by the Compton effect. This can also bring about a secondary emission of braking radiation.
The energy carried by the photons is then redistributed into the resist through the successive collisions of the photoelectrons that have been generated, determining partial exposure of zones that should have remained hidden. This effect is commonly known as “shadow effect” and its extent is a function of the electron range, i.e., of the mean distance covered by the photoelectron from its point of origin to the point where it has zero kinetic energy. The combination of the diffraction effects and of the shadow effect imposes a limit on the resolution that can be achieved with traditional lithography or more correctly on its patterning capability, understood as capacity of the used technique to reproduce a given pattern in given lighting conditions.